A laser dicing method for cutting an object such as a wafer into multiple chips by using a laser beam has been developed. For example, a focus of the laser beam is adjusted at an inside of the object, and the laser beam is irradiated on the object so that a reforming region is formed in the object. The reforming region is formed by a multi-photon absorption effect, and includes, for example, a crack, a melting region, or a region having a refraction index different from the object. The reforming region provides a starting point of cutting. The reforming region is formed along with a cutting line of the object such that the reforming region is disposed inside of the object by a predetermined distance from a surface of the object as an incident surface of the laser beam. The object is cut along with the reforming region so that the object is cut into multiple chips. This method is disclosed in, for example, US Patent Application Publication No. 2006-0160331.
Another method for forming multiple reforming regions is disclosed in JP-A-2002-205180. Specifically, a focus point of a laser beam is adjusted at an inside of an object. Then, the laser beam is irradiated on the object, so that the reforming region is formed at the inside of the object along with a cutting line. Further, the focus point in an incident beam direction of the laser beam is changed so that multiple reforming regions are formed along with the incident beam direction, which is perpendicular to the surface of the object. In this case, since multiple reforming regions are arranged along with the incident beam direction, the number of staring points of cutting increases. Thus, even when the object has a large thickness, the object is easily cut.
Further another method for cutting an object by using a laser beam is disclosed in US Patent Application Publication No. 2006-0011593. In this method, the object has a plate shape, and includes a substrate or the like. An expansible film is formed on one side of the object, and the laser beam is irradiated on the other side of the object so that the other side provides an incidence surface of the laser beam. A focus point of the laser beam is adjusted at an inside of the object, and then, the laser beam is irradiated on the object. Thus a reforming region is formed at the inside of the object by a multi photon absorption effect. In this case, the reforming region includes a melting process region. The reforming region provides a starting point of cutting along with a cutting line, and is disposed inside of the object by a predetermined distance from the other side of the object. Then, the expansible film is expanded so that the object is cut into multiple parts along with the starting point of cutting. In this case, the parts are separated one another. In this case, after the reforming regions are formed at the inside of the object, the expansible film is expanded. Thus, a tensile stress is applied to the reforming regions appropriately, so that the object is accurately and easily cut from the staring point of cutting with a comparatively small force.
On the other hand, a blade dicing method is used for cutting and separating a wafer by rotating a dicing blade having a diamond abrasive grain. In this case, a process time for cutting the wafer is comparatively short. Therefore, a throughput (i.e., productivity per an unit time) is high, and the blade dicing method is suitable for mass production.
However, the blade dicing method has the following difficulties.
(1) It is necessary for the blade dicing method to have a margin for a thickness of the dicing blade. Thus, the number of chips cut from the wafer is reduced by the margin. Thus, a product yield of the chips is limited.
(2) When the wafer is cut by the dicing blade, abrasive heat and cutting scrap are generated. Accordingly, it is necessary to cool and wash a cutting portion of the wafer. Specifically, cooling and washing water is introduced to the cutting portion of the wafer.
In this case, when the chip includes a construction having a movable portion, movable performance of the chip may be reduced in a case where the cutting scrap and/or the cooling water penetrates into the construction. Here, the construction having the movable portion is, for example, a sensor such as a pressure sensor, an acceleration sensor and an ultrasonic sensor having a piezo electric element or a capacitor or a micro machine, which is formed by using a MEMS (i.e., micro electro mechanical system) technique.
To protect the construction from introducing the cutting scrap and the cooling water thereinto, a protection layer is stacked on a semiconductor layer, so that a semiconductor substrate has a double-layer structure. In this case, if a clearance is formed between the protection layer and the semiconductor layer, the cutting scrap and/or the cooling water may penetrate into the clearance. Thus, the performance of the movable portion is reduced. Here, a technique for forming a multi-layer substrate is, for example, a bonding method with using a silicon series bonding member, a bonding method with a low melting point glass, a direct bonding method, and an anodic bonding method.
FIGS. 11-13 show a wafer 10 and a chip 11 according to a related art of the present disclosure. Specifically, the wafer 10 includes multiple chips 11, and each chip 11 has a construction 12 formed by a MEMS technique.
The wafer 10 has a thin disk shape, and multiple chips 11 having the same construction are formed on one side 10b of the wafer 10. The chips 11 are arranged on the wafer 10 to be a grid shape. A cutting line K is formed between the chips 11. The wafer 10 is to be cut along with the cutting line K.
The wafer 10 has a SOI structure, which is prepared by bonding together. Specifically, a silicon substrate layer 19 made of silicon single crystal, a buried oxide layer 18 (i.e., BOX layer), and a SOI layer 13 made of silicon single crystal are stacked in this order from a bottom to a top of the wafer 10. The BOX layer 18 as an insulation layer is formed on the SOI layer 13 so that the SOI structure is provided.
The wafer 10 having the SOI structure is formed such that two wafers having an oxide film on one surface thereof are bonded each other with the oxide film therebetween, and then, one of wafers is ground so as to have a predetermined thickness. The one surface of each wafer is defined as a bonding surface and is a mirror finished surface. The ground wafer provides the SOI layer 13, the other wafer provides the silicon substrate layer 19, and the oxide film provides the BOX layer 18.
Each semiconductor chip 11 in the wafer 10 includes a construction 12, an electrode pad 14, a trench 15, a cap portion 16, a seal member 17 and the like, which are formed by using a MEMS technique.
The construction 12 having a movable portion such as a sensor element and a micro machine is formed in the SOI layer 13, in which an impurity is diffused with high concentration.
The electrode pad 14 is made of a metallic film disposed on a surface of the SOI layer 13, and led from the construction 12. The metallic film is formed by a PVD (i.e., physical vapor deposition) method, a printing method or the like.
A trench 15 is formed around a part of the SOI layer 13 under the electrode pad 14 and around another part of the SOI layer 13 providing the construction 12. The trench 15 electrically separates a wiring in the SOI layer 13 and other parts of the wafer 10. This separation is defined as element separation. Thus, each part constituting the construction 12 is movably independent from each other.
The cap portion 16 made of bulk silicon such as poly crystal silicon, amorphous silicon and single crystal silicon protects and covers the construction 12. The cap portion 16 is bonded to the surface 10b of the wafer 10 with a bonding member 20 such as a silicon series bonding member and a polyimide series bonding member. Here, to improve movement of the movable portion in the construction 12, a clearance 30 is formed between the inner wall of the cap portion 16 and the construction 12.
The cap portion 16 formed in each chip 11 is integrated continuously on a whole surface of the wafer 10. Thus, only a part is exposed on the surface 10b of the wafer 10, the part not covered with the cap portion 16. Specifically, the part is disposed around the electrode pad 14.
An outer periphery 10h of the wafer 10 on the surface 10b is covered with the cap portion 16.
The cap portion 16 is stacked on the surface 10b of the wafer 10 so that the wafer 10 has double-layered structure. The wafer 10 with the cap portion 16 is separated and cut in a stacking direction (i.e., a thickness direction of the wafer 10) so that multiple chips 11 are obtained.
The seal member 17 is embedded in the trench 15, and the seal member 17 made of plastic material having insulation property is connected to the cap portion 16. The seal member 17 seals the cap portion 16.
When the wafer 10 is cut and separated into multiple chips 11 by the blade dicing method, cooling and washing water is introduced to the cutting portion of the wafer 10 for cooling and washing the cutting portion. In this case, the water and a cutting scrap of the wafer 10 and/or the cap portion 16 may penetrate into the construction 12 through the clearance 30 between the seal member 17 and the trench 15 or the cap portion 16.
When the cutting scrap and/or the water penetrate into the construction 12, performance of the construction 12 is reduced. Thus, yielding ratio and quality of the chips 11 separated from the wafer 10 are reduced.
It is considered that sealing with the seal member 17 is improved, i.e., tightened for preventing the water and the cutting scrap from penetrating into the construction 12. However, in this case, a manufacturing cost may increase.
Further, even when the sealing with the seal member 17 is improved, it is difficult to prevent the water and the cutting scrap completely from penetrating into the construction 12.
In the laser dicing method described above, it is not necessary for the wafer 10 to have a margin for a thickness of the dicing blade, and no water is used in the laser dicing method. Thus, the above difficulties of (1) and (2) are avoidable.
When the wafer are made of a single layer substrate, or when the wafer has no construction formed by the MEMS technique, the wafer is easily and accurately cut and separated by the laser dicing method.
However, it is difficult for the wafer 10 having the cap portion or a wafer having a double-layered structure to form the reforming region in the wafer 10 and the cap portion 16 preferably.
This is because optical properties among the wafer 10, the cap portion 16 and the bonding member 20 are different from one another. Specifically, refraction indexes of the wafer 10, the cap portion 16 and the bonding member 20 in relation to the laser beam L are different from one another. Accordingly, a part of the laser beam L is reflected by a boundary between the wafer 10 and the bonding member 20 or a boundary between the bonding member 20 and the cap portion 16. Thus, the reflected laser beam and the incident laser beam interfere with each other so that they are cancelled each other. The energy of the laser beam L is much attenuated at a deep place apart from the incident surface of the laser beam L. At the deep place, the energy of the laser beam L is insufficient for generating the multi-photon absorption effect, and therefore, it is difficult to form the reforming region sufficiently.
Here, the bonding member 20 bonds the cap portion 16 and the wafer 10, and may not be disposed on the cutting line K in some case. In this case, the bonding member 20 is disposed outside of the construction 12, and further disposed inside of the cap portion 16. In this case, since optical properties among air between the cap portion 16 and the wafer 10, the cap portion 16 and the wafer 10 are different, so that refraction indexes among the air, the wafer 10 and the cap portion 16 in relation to the laser beam L are different from one another. Thus, a part of the laser beam L is reflected by a boundary between the wafer 10 and the air or a boundary between the air and the cap portion 16. Therefore, it is difficult to form the reforming region in the wafer 10 and the cap portion 16 sufficiently.
Further when the wafer 10 has the double-layered structure, optical properties among layers are different, so that refraction indexes among the layers are different from each other. Thus, a part of the laser beam L is reflected by a boundary between the layers. Therefore, it is difficult to form the reforming region sufficiently.
When the reforming region is not formed sufficiently in the wafer 10, an unwanted crack may be generated in the wafer 10 in case of cutting and separating. Thus, it is difficult for the wafer 10 to cut and separate along with the cutting line K. Thus, the yielding ratio and quality of the chips 11 are reduced.
Further, it takes a long time to form the reforming region in the wafer 10 having the cap portion 16 or the wafer having the double-layered structure by using the laser dicing method. Thus, the throughput of the chips 11 with the laser dicing method is lower than that with the blade dicing method. Accordingly, the laser dicing method is not suitable for mass production.
Thus, it is required to provide a method for cutting a wafer with high throughput and low cost, the method providing to prevent foreign substance such as cutting scrap from penetrating into a chip. Further, it is required to provide a wafer or a chip with high throughput and low cost, the wafer or the chip having no foreign substance.